Materials that combine ultra-low density with the desirable characteristics of metals have been an object of technical development for decades, and a variety of metals and alloys are commercially available in various cellular forms. Cellular structures made from shape-memory alloys (SMAs) are particularly intriguing for their potential to deliver shape memory and/or superelasticity in a lightweight material. While porous forms of NiTi have been made [Lagoudas et al., 2001], the difficulty of joining Nitinol to itself has prevented the realization of built-up cellular honeycombs from NiTi-based SMAs
Even when conventional strength and stability characteristics are all that is sought, metallic foams and honeycombs, with their light-weight, high specific stiffness, and well-developed energy absorption characteristics, are of obvious utility [Gibson and Ashby, 1997; Ashby, et. al., 2000]. In particular, Papka and Kyriakides [1994, 1998] presented interesting in-plane crushing experiments of hexagonal aluminum honeycombs. These showed an initial stiff response, followed by a plateau where crushing continued at nearly constant load. The plateau was associated with localized deformation of particular rows of cells. Shear-like bands propagated as the honeycomb densified, and the plateau ended as mutual contact of the cell walls caused the load to rise steeply. Of course, the aluminum honeycombs in these experiments suffered permanent deformation.
Some attempts to produce porous SMAs by hot isostatic pressing of powders [Lagoudas, et al., 2001, Thangaraj, et al., 2000, Li, et al., 2000] have achieved relative densities as low as 30%, but the irregular pore shape in these materials causes stress concentrations that severely degrade mechanical properties. More recently, a Nitinol-based material with a more regular, open-cell foam topology, and a relative density below 5%, was reported [Shaw, et al., 2002; Grummon, et al., 2003]. These materials were realized using a powder metallurgy technique and a polymeric foam precursor, and were shown to possess the martensitic transformation characteristics of SMAs. Unfortunately, embrittlement by interstitial contaminants prevented a useable superelastic response.
Honeycomb structures built up from wrought SMAs are a viable alternative to foamed or porous metals if a method can be found to join individual corrugated or dimpled sheets or strips. The necessary joining step to create an open topology must not only provide a robust metallurgical bond, but must also be derivable from a simple, clean, and cost-effective batch process. Given the range of potential applications, the bond should additionally have high corrosion resistance, good thermal stability, and should contain only biocompatible phases. While a few specialized techniques for soldering and welding Nitinol have been developed over the years, until now no low-cost joining method capable of producing tough metallurgical bonds in complex multiple-contact structures, such as honeycombs or spaceframes, has been available.
Recent manufacturing advances enabled the creation of metallic honeycombs and meshes, which are of increasing technological and economic importance. As used herein, a honeycomb or a mesh is a material having an extreme degree of free space, i.e., less than 10% dense, preferably less than 5% dense, compared to a simple porous material that typically is 40% to 90% dense.
Metallic shapes can be manufactured either as two-dimensional honeycombs or as three-dimensional open- or closed-cell configurations, depending on the manufacturing process. Open-cell configurations are a network of thin ligaments joined at nodes that act as beam/torsion elements. Closed-cell configurations, by contrast, are composed of intersecting polyhedra with the material forming thin shell elements. Both types of forms possess the properties of light weight and high specific stiffness, making the foams suitable for a variety of structural applications, such as sandwich cores.
Manufacturing techniques currently are being developed that can produce complex geometries, and in the case of sandwich cores, include integral face plates. In overload conditions, the geometries can provide efficient energy absorption through the mechanisms of crushing and densification. These honeycomb and mesh structures initially are stiff in compression, but once a critical load is reached, they crush by buckling and collapse of the thin ligaments at a relatively constant load. In addition, providing cooling flow of gaseous or liquid materials through an open-cell configuration provides an effective means of heat transfer, thereby making the metallic structure multifunctional.
A variety of manufacturing approaches have been used to produce honeycomb and mesh metallic configurations from conventional alloys, including inert gas blowing, mechanical stirring, and the introduction of outgassing agents into the molten metal before freezing. Aluminum is particularly suited for use in these processes because of its low density and low melting point. Investment casting has also been used to manufacture metallic honeycombs and meshes. In this process, the molten metal is poured into the interstices of a particulate material, like a water-soluble salt, which then is flushed away after the metal freezes.
One objective of the above-identified methods of manufacturing metallic structures, such as honeycombs and meshes is to provide a class of materials that combines the properties of a shape memory alloy (SMA) with extremely sparse topology. SMAs, such as NiTi (or nitinol), exhibit two remarkable strain recovery properties in wrought form, i.e., the shape memory effect and superelasticity. The first property refers to an ability of an SMA to recover from large mechanically induced strains (i.e., up to 8%, e.g., in extant SMA structures) by moderate increases in temperature. The latter properties refers to the rubber-like, hysteretic strain recovery in relatively high temperature regimes. In each case, the underlying mechanism is a reversible martensitic transformation between solid-state phases that can be induced by changes in temperature or stress. An important SMA is near equiatomic NiTi, which also has excellent structural properties, corrosion resistance, and long-term biocompatibility, making NiTi an attractive choice for many structural applications.
Shaw et al., in “Proceedings of 2002 ASME International Mechanical Engineering Congress and Exposition,” Nov. 17-22, 2002, New Orleans, La., pages 1-10, incorporated herein by reference, discloses the production of a light (5% dense) NiTi structure having a more regular open-cell structure, which provides an improved specific stiffness and improved strain recoverability. The Shaw et al. publication also provides a review of recent experimental work on monolithic NiTi.
SMA honeycomb and mesh materials, therefore, have the potential to act as adaptive structural elements that respond to changes in external loads and the thermal environment. Also, the superelastic properties of SMAs can be used to create lightweight, reusable energy-absorbing materials, or highly damage-tolerant, self-repairing materials. A variety of applications are envisioned for such SMA honeycomb and mesh materials, like impact-tolerant armor, vibration-isolation elements, adaptive skins, and control surface elements, for example. The biocompatibility and extreme porosity of a NiTi honeycomb or mesh also lends itself to use in biomedical devices and implants.
Although prior investigators prepared metallic foams, such as nitinol foams, no method currently is available to effectively join individual nitinol shapes, such as tubes and wires, to provide a structure containing joined nitinol, or other SMA, elements that is sufficiently strong for practical applications. The present invention is directed to a method of joining individual metallic SMA elements, such as honeycomb and mesh elements into built-up structures for effective use in practical applications.